1. Field of the invention
The present invention relates to a bead inductor, particularly a bead inductor having a coil disposed in a bead-like core.
2. Description of Related Art
FIG. 10 is an illustration showing an example of a conventional bead inductor. A bead inductor 1 includes a rectangular-parallelepiped core 2, for example. A coil 3 comprising a wound metal wire is disposed in the core 2. Ends of the coil 3 extend out to the opposite ends of the core 2. In other words, as shown in FIG. 11, the core 2 is formed by an axial portion 4 in the central portion thereof and a peripheral portion 5. The coil 3 is disposed on the periphery of the axial portion 4.
The peripheral portion 5 is disposed on the periphery of the coil 3. Furthermore, a metal cap 6 is disposed on the opposite ends of the core 2. The coil 3 is electrically connected to the metal cap 6. The metal cap 6 functions as a terminal for connecting with an external circuit.
In order to produce such a bead inductor 1, magnetic powder, such as a ferrite, is kneaded into resin, and a mixed material is produced. The axial portion 4 is formed by extrusion molding using the mixed material. The coil 3 is formed by winding a metal wire around the axial portion 4. Furthermore, the peripheral portion 5 is formed by extrusion molding using the mixed material and is formed on the periphery of the axial portion 4 after the coil 3 is formed. The bead inductor 1 is completed by fixing the metal cap 6 on the opposite ends of the core 2.
In the bead inductor 1, when a signal transmits through the coil 3, a flux is generated at the periphery of the coil 3, in other words, at the axial portion 4 and the peripheral portion 5. At this time, as shown in FIG. 12, a large inductance is produced at the inside and outside of the coil 3 by the axial portion 4 and the peripheral portion 5 having high permeability xcexc. Therefore, high frequency noise can be eliminated by transmitting a signal to the bead inductor 1. In addition, the cylinder in FIG. 12 shows the coil 3.
However, if the content of the magnetic powder kneaded into the resin increases, the molded structure using mixed material becomes brittle. Even if the axial portion is formed by extrusion molding, it becomes difficult to wind the coil and to store the axial portion as a half-finished product. Furthermore, when the peripheral portion is formed by extrusion molding on the periphery of the axial portion after the coil is formed, breaks and cracks occur in the axial portion. As a result, it becomes difficult to reliably produce a non-defective bead inductor using mass production processes. To avoid such an undesirable result, reducing the quantity of a magnetic powder in the core may be attempted. However, since the axial portion is the portion which the flux flows around, it is desirable that the permeability of the axial portion is high.
In order to overcome the problems described above, preferred embodiments of the present invention provide a bead inductor having reliable electrical characteristics and constructed so as to be capable of being manufactured easily using mass production processes.
According to one preferred embodiment of the present invention, a bead inductor includes a core made of a magnetic material and a coil made of a conductive material and disposed in the core, wherein a central portion inside of the coil is made of a material having high strength.
According to another preferred embodiment of the present invention, a bead inductor includes a core made of a magnetic material and a coil made of a conductive material disposed in the core, wherein a central portion inside of the coil is a cavity.
When a current flows in the coil, flux occurs in the core. At this time, not much of the flux is generated in the central portion of the coil and the flux concentrates in the vicinity of the coil. Thus, it is not necessary to construct the central portion of the coil where the flux is low with a high permeability material. Instead, the central portion can be formed with a high strength material. Moreover, if at the time of molding, a required strength of the axial portion can be secured, the bead inductor can be mass-produced reliably. Therefore, even if the central portion of the coil is removed after molding, the proper operation and function of the bead inductor can be secured.
The above-described elements, features, and advantages of the present invention will be further clarified by the detailed descriptions in the description of the preferred embodiments which will be described below by referring to the drawings.